The Margin of error is the statistical way of calculating the random error in a given sample. It tells us to what extent the survey is reliable!. It is expressed in percentage.
Margin of Error formula is given by
Where $P$ is the probability of proportion of population,
$N$ gives the size of the population,
$n$ is the size of the sample.
Suppose you are having the set of children in which there are 60% of them participating in sports. If the marginal error is 5% it means that there is possibility of 55% - 65% people participating in sports. Thus the marginal error tells you how accurate the results are!. The greater the marginal error, lesser will be the accuracy.
Marginal Error Calculator is a online tool that gives the marginal error of the given survey if the size of the population N, probability p and sample size n are entered in blocks provided.